1. Field of the Invention
The present invention relates to a display device having a pixel array unit in which pixel circuits (referred to also as pixels) including an electrooptic element (referred to also as a display element or a light emitting element) are arranged in the form of a matrix, and a driving method of the display device, and particularly to an active matrix type display device that is formed by arranging pixel circuits having an electrooptic element changing in luminance according to the magnitude of a driving signal as a display element in the form of a matrix and which has an active element in each pixel circuit, display driving being performed in a pixel unit by the active element, and a driving method of the active matrix type display device.
2. Description of the Related Art
There are display devices that use an electrooptic element changing in luminance according to a voltage applied to the electrooptic element or a current flowing through the electrooptic element as a display element of a pixel. For example, a liquid crystal display element is a typical example of an electrooptic element that changes in luminance according to a voltage applied to the electrooptic element, and an organic electroluminescence (hereinafter described as organic EL) element (organic light emitting diode (OLED)) is a typical example of an electrooptic element that changes in luminance according to a current flowing through the electrooptic element. An organic EL display device using the latter organic EL element is a so-called emissive display device using a self-luminous electrooptic element as a display element of a pixel.
The organic EL element is an electrooptic element using a phenomenon of light emission on application of an electric field to an organic thin film. The organic EL element can be driven by a relatively low application voltage (for example 10 V or lower), and thus consumes low power. In addition, the organic EL element is a self-luminous element that emits light by itself, and therefore obviates a need for an auxiliary illuminating member such as a backlight needed in a liquid crystal display device. Thus the organic EL element can be easily reduced in weight and thickness. Further, the organic EL element has a very high response speed (for example a few μs or so), so that no afterimage occurs at a time of displaying a moving image. Because the organic EL element has these advantages, flat-panel emissive display devices using the organic EL element as an electrooptic element have recently been actively developed.
Recently, the development of an active matrix system, which controls a pixel signal supplied to a light emitting element within a pixel by using an active element, for example an insulated gate field effect transistor (generally a thin film transistor (TFT)) similarly provided within the pixel as a switching transistor, has been actively underway.
In this case, in making an electrooptic element within a pixel circuit emit light, the switching transistor takes an input image signal supplied via a video signal line in a storage capacitor (referred to also as a pixel capacitance) provided at the gate terminal (control input terminal) of a drive transistor, and supplies a driving signal corresponding to the taken input image signal to the electrooptic element.
In the organic EL display device using the organic EL element as electrooptic element, because the organic EL element is a current-driven type element, the drive transistor converts the driving signal (voltage signal) corresponding to the input image signal taken in the storage capacitor into a current signal, and supplies the driving current to the organic EL element.
In a current-driven type electrooptic element typified by the organic EL element, a different driving current value means a different light emission luminance. Hence, for light emission at stable luminance, it is important to supply stable driving current to the electrooptic element. For example, driving systems for supplying driving current to the organic EL element can be roughly classified into constant-current driving systems and constant-voltage driving systems (the systems are well known techniques, and therefore publicly known documents thereof will not be presented).
Because the voltage-current characteristic of the organic EL element has a steep slope, when constant-voltage driving is performed, slight variations in voltage or variations in element characteristic cause great variations in current and thus bring about great variations in luminance. Hence, constant-current driving, in which a drive transistor is used in a saturation region, is generally used. Of course, even with constant-current driving, changes in current invite variations in luminance. However, small variations in current cause only small variations in luminance.
Conversely, even with the constant-current driving system, in order for the light emission luminance of an electrooptic element to be unchanged, it is important for a driving signal written to a storage capacitor according to an input image signal and retained by the storage capacitor to be constant. For example, in order for the light emission luminance of an organic EL element to be unchanged, it is important for a driving signal corresponding to an input image signal to be constant.
However, the threshold voltage and mobility of an active element (drive transistor) driving the electrooptic element vary due to process variations. In addition, the characteristics of the electrooptic element such as the organic EL element or the like vary with time. Variations in the characteristics of the active element for such driving and variations in the characteristics of the electrooptic element affect light emission luminance even in the case of the constant-current driving system.
Thus, various mechanisms for correcting luminance variations caused by variations in the characteristics of the active element for the above-described driving and the electrooptic element within each pixel circuit are being studied to uniformly control the light emission luminance over the entire screen of a display device.
For example, a mechanism described in Japanese Patent Laid-Open No. 2006-215213 (referred to as Patent Document 1 hereinafter) as a pixel circuit for an organic EL element has a threshold value correcting function for holding the driving current constant even when there is a variation or a secular change in threshold voltage of the drive transistor, a mobility correcting function for holding the driving current constant even when there is a variation or a secular change in mobility of the drive transistor, and a bootstrap function for holding the driving current constant even when there is a secular change in current-voltage characteristic of the organic EL element.